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Microbial decomposition of organic matter and nutrient regeneration in natural waters and sediments: Report on literature study

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Preface

This report deals witb the result of an extensive literature research with respect to the nature of mineralization processes in natural waters and their sediraents.

The research had to yield qualitative and quantitative information about this subject, which will be used in the construction of a model, that

describes nutriënt exchange between sediment and overlaying water. The need of such a model became evident during nutriënt modelling activities with the chemical model GHARON for several Dutch fresh water lakes. It appeared, that bottom nutriënt loads could not be modelled in a simple way, because mineralization processes are very complex and greatly affect bottom loads. This research is part of a raultidisciplinary project, called Water Basin Model (WABASIM)J the project is carried out in close cooperation between the Environmental Division of the Delta Department and the Environmental Hydraulics Branch of the Delft Hydraulics Laboratory.

The literature research was perforraed by ir. J.G.C. Smits of the Delft Hydraulics Laboratory.

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Contents (continued) page 3.3 The nitrogen cycle 33 3.1.1 Primary production of organic-N 33 3.J.2 Mineralization in water 33 3,1.3 Mineralization in sediment 33

3.2 Agmonification 36 3.2.1 Amroonification in natural water 36 3.2.1.1 Siraple first-order model 37 3.2.1.2 First-order model with a refractory part 38 3.2.1.3 More complex first-order models 39 3.2.1.4 A model for the calculation of the refractory part 40 3.2.2 Ammonification in sediments of natural water 41 3.2.2.1 Ammonif ication rates in sediment 41 3.2.2.2 Relation of carbon mineralization and ammonification 45 3.2.2.3 Models for nitrogen release from sediment.... 45

3.3 Nitrification 47 3.3.1 Rates of nitrification in natural water 48 3.3.2 Rates of nitrification in sediment of natural waters 49 3.3.3 The Monod model and its parameter values 50 3.3.4 Oxygen limitation 54 3.3.5 Inhibition by pH, substrate and product 54 3.3.6 Temperature dependence of rate constants and saturation

constants 55

3.4 Denitrif ication 56 3.4.1 Overall rates of denitrification in sediment and water.... 57 3.4.2 Monod model parameter values for denitrification in water

environment 57 3.4.3 Sediment denitrification models 57 3.4.3.1 Models: rates as a function of interstitial water nitrate

concentration 57 3.4.3.2 Models: rates as a, function of oyerlaying water nitrate

. concentration , , , 59 3.4.4 Temperature dependence of rates and rate constants 62 Literature 64

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4 Mineralization. of organic phosphorus... 71

4.1 The Phosphorus cycle. , 72

4.1.1 Primary production 72

4.1.2 Mineralization 72

4.2 Dissolyed inorganic phosphorus regeneration from organic

matter, prodttced by algae 74

4.2.1 Phosphorus regeneration in water,., 74

4.2.1 . J First-order model 74

4.2.1.2 More complex models for phosphorus regeneration... 77

4.2.1.3 A model for the calculation of the refractory fraclion.,.. 78

4.2.2 Phosphorus regeneration in sediment.. 80

4.2.2.J Overall phosphorus release rates ,,, 80

4.2.2.2 First-order model for phosphorus regeneration 81

4.2.2.3 Models for phosphorus release of sediments 81

4.2.2.4 A stoichiometric model for phosphorus regeneration in

se-d iment s , 83

Literature 86

5 Regeneration of dissolved silica 89

5.1 The silicon cycle. 90

5.2 Dissolution of biogenic amorphous silica in water 91

5.2.1 Factors, affecting the dissolution rate and solubility.,.. 91

5.2.2 The first-order model for silica regeneration in water.,,. 92

5.3 Dissolution of biogenic amorphous silica in sediment 94

5.3.1 Factors, affecting the dissolution rate and solubility.,,, 94

5.3.2 • Overall silica release rates of and saturation

concentra-tions in sediments 94

5.3.3 Models for silica regeneration in sediments... 95

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Contents (continued)

page

Literature , 98

6 Sonte relevant processes with respect to sulfur components. 100

6.1 The sulfur cycle 101

6.1.1 Primary production 101

6.1.2 Reduction and oxidation processes in water 101

6.1.3 Reduction and oxidation processes in sediment..,. 102

6.2 Sulfate-reduction ;. .. . 104

6.2.1 Overall sulfate-reduction rates... 105

6.2.2 The sulfate concentration as limiting factor 106

6.2.3 The organic matter content as limiting factor, 106

6.2.4 Models for sulfate-reduction in sediments.... 106

6.2.5 Temperature dependence of sulfate-reduction....,.., 108

6.3 Biochemical oxidation of sulfide.... 109

6.3.1 Rates of aerobic biochemical sulfide oxidation.... 110

6.3.2 Rates of anaerobic fototrophic sulfide oxidation 110

6.4 ' Chemical oxidation of sulfide 112

6.5 Reaction of sulfur compounds with iron ions in the

sedi-ment , ....,..., , 113

Literature..., 114

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simple inorganic substances, like carbon dioxide, water, ammonium and phos-phate, The latter two are nutrients for primary producers like algae. Mine-ralization is a key process in the biocycle of a number of chemical elements, of which the most important are carbon, nitrogen, phosphorus-, sulfur and si-licon.

Four major types of microbial breakdown processes can be distinguished. They differ within tnany respects, but the most evident one is the use of differ-ent electron acceptors in the oxidation of organic matter. The electron acceptors are subsequently oxygen, nitrate, sulfate and carbon dioxide. Besides the micro-organisms, that use external electron acceptors, there are micro-organisms, that can do without them. They are called fermentative micro-organisms and simply split complex organic matter into carbon dioxide

(fully oxidized carbon) and low molecular organic components like alcohols and fatty acids (relatively more reduced carbon), which are the main sub-strates for sulfate reducers and methanogens, Although fermentative degrada-tion always accompanies the anaerobic degradadegrada-tion processes, not much at-I tention is paid to it, because the terminal degradation process in natural

water systems always is one of the electron acceptor demanding types, that mainly rule the chemical environment.

I Mineralization strongly affects the availability of nutrients for. primary producers as well as the chemical enyironmental conditions in natural waters I and their sediments. The rate, with which and the extent, in which minerali-• zation takes place in a natural water system determines if anaerobic reduced

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conditions occur in the upper sediment layer and the lower part of the

overlaying water or not. The more reduced the sediment is the higher the equilibrium concentrations of nutriënt, phosphate in particular, in the I interstitial water may be, It will be clear, that the result of a steeper

gradiënt of the nutriënt concentration in the upper layer of the sediment I. will be a higher bottom nutriënt load to the overlaying water. This again

means a higher availability of nutrients to primary producers like algae, in B other words an increase of eutrophication.

Now we have touched the ultimate motive for the production of this report.

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Mineralization is a yery complicated system of processes, the rate of which

is a function of temperature and supply of organic matter and electron acceptors and strongly affects the extent of eutrophication. So proper

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nutriënt modelling.will not be possible, if mineralization, particularly in the sediment, and mass transport between sediment and water are not includ-ed in a sufficiently detailinclud-ed way. In order to be able to construct a model, which describea mineralization in sediment and mass transport between sedi-ment and overlaying water, one needs profound understanding of the nature of mineralization processes and sufficient quantitative information with respect to kinetica. Both must be supplied by this report, which is the pre-cipitate of a literature research concerning mineralization.

The report is partitioned in six chapters. Chapter two, threë and four de-scribe the mineralization processes and some additional relevant microbial processes with respect to subsequently carbon, nitrogen and phosphorus. The fifth chapter concerns the regeneration of dissolved silica from biogenic opaline silica, which is not biologically mediated but a chemico-physical process, Silicon is included because sulfate is one of the electron accep-tors. The report covers mineralization in fresh as well as salt water

systems, between which distinction has been made if necessary, For all pro-cesses, mathematical descriptions and parameter values, found in literature, are given.

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and mineralization of algae in natural waters, in order to get a picture of the interrelation of the relevant processes. Sections 2.3 and 2.4 deal with aerobic and anaerobic mineralization.

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1,1 _ The carbon cycle

Three phases have been distinguished with respect to the carbon cycle: pri-, mary production, by algae, mineralization in water and mineralization in se-diment. Secondary production and primary production by organisms, other than algae, are left out of consideration. It is assumed, that these organisins quantitatively play a minor role in Dutch natural waters,

There is no essential difference between carbon cycles in fresh and salt water. If differences are relevant however, they will be discussed, but we will start from the fresh water carbon cycle. Figure 2.1 gives a picture of this cycle (page 8 ) .

2.1.3 Frimary production by algae

The carbon cycle takes a start with the primary production, In this process organic matter, biomass, is produced by photosynthesis out of simple inor-ganic substances like carbon dioxide, ammonium or nitrate, phosphate and sulfate. Sugars, amino acids and fatty acids, synthetized in this way, are being used for the construction of cell matter and for the production of maintenance energy. In the latter case the organic carbon is oxidized to

carbon dioxide again,

Algae fixate carbon dioxide in light and dark. This process is more or less independent of photosynthesis and can yield maximally 10% of total produc-tion.

A part of the organic matter, normally some percents of total production, is immediately excreted by the algae into the surrounding medium. To a limited extent algae are able to ingest the excreted organic substances, like amino acids and fatty acids, again 0,2,3,4].

2.1.2 Mineralization in water

A small part of the algae biomass is consumed by zoöplankton and fish. The bigger part dies before it comes that far. Dead algae account for the bigger part of the dead organic matter, called detrites, that is present in natural waters.

Immediately after death of a cell fast autolysis takes place [5]. Katabolic enzyraes, present in the cell, break down a part of cell matter to small fragments, like peptides, amino acids, fats and fatty acids, that, after rupture of the cell wall, leak away into the surrounding medium. The dead

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cell ma.y loose a substantial percentage (about 20%) of its matter in this way in a very short time. Heterotrophic aerobic bacteria decompose the or-ganic matter, dissolved in tbe water and coming from excretion and autolysis, to carbon dioxide and nutrients [ï ,2,6,7,8]. Particulate and dissolved organ-ic matter also aggregate to large polymer molecules, that become more and more resistant to microbial decomposition, and settle at the sediment in

course of time, in which it is burried.

The dead algae cells and their fragments settle at the sediment too. During settling the organic matter is broken down partially by bacteria, that are attached to the particulate matter, to smaller fragments and to carbon

dioxide and nutrients in the end. Bacteria do not only decompose particulate organic matter, they also produce it by means of synthesis of their own bio-mass and polymer extracellular matter, like polysaccharides, This

contribu-tion to the total amount of settling particulate matter seems tö be substan-tial [2,8j, Living bacteria themselves mostly do not account for more than 1% of the particulate matter [9].

Allochtonous dissolved and particulate organic matter, biogenic or not, is involved in the carbon cycle in the satne way as the organic matter, coming

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from algae. Mostly it forms a relatively small contribution to the totalamount of organic matter present in natural waters, Beeause the mean settling time in Dutch lakes varies from 4 to 10 days and I complete decomposition takes several tens to hundreds of days, the biggest

part of the decompositon process takes place on and in the sediment.

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2.1.3 Mineralization in the sediment

Due to bioturbation and the accumulation of the sediment, settled organic

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matter is burried in the sediment. As the organic matter is containedlonger (and beeause of that deeper) in the sediment, it becomes more and more resistant to biodegradation due to chemical changes. In the end a non-I degradable fraction of 30 to 40% of the original algae matter, so it seems, remains [5,9]. This fraction contains humic acids. and such-like substances. I The way miniralization takes place in the sediment strongly depends of the

conditions in the overlaying water. If the overlaying water is mixed homo-geniously and aerobic, mineralization proceeds in the following way.

In the upper layer of the sediment oxygen will be present and used for the oxidation of organic matter. So in this very thin layer aerobic degradation occurs, A large group of -many different heterotrophic aerobic bacteria

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pro 6 pro

-duces extracellular enzyroes, that hydrolize the large molecules of cell matter like proteins, carbohydrates (cellulose), lipids and DNA to

pep-tides, amino acids, sugars (glucose), fatty acids and ribonuclelc acids. These small molecules are being dissimilated with oxygen, the electron

acceptor, intracellular by the same bacteria to carbon dioxide and tiutrients. The aerobic layer never is more than a few millimeters thick in the hyper-eutrophic Dutch lakes [l2].

If the oxygen concentration drops to zero and the redox potential has been reduced to +300 mV or less for that reason, nitrate will replace oxygen as an electron acceptor. The denitrifying layer is, dependent of the nitrate concentration in the oyerlaying water, several millimeters or centimeters thick. Denitrifyers are heterotrophic organisms (Pseudontonas, Alkaligenes, Bacillus, Flavobacterium) except Thiobacillus denitrificans. This bacterium is autotrophic and receives its energy from the oxidation of sulfide with nitrate [.12]. The aerobic and denitrifying layers are indicated together as the oxidized layer. The layer, that lies underneath, is called the reduced layer. After depletion of nitrate respectively manganese (IV), iron (III) and sulfate are being used as an oxidator. When sulfate reduction begins to occur the redox potential has been reduced to +100 mV. Sulfate reduction by I amongst others Desulfovibrio species is playing a much less important role

in fresh water sediment than in marine sediment, because sulfate

concentra-I

tions in fresh water are small (< 20 ppm) compared to sulfate concentrations

in salt water (+_ 2000 ppm) [j3J. For that reason there will not be a clear cut sulfate reduction layer in fresh water sediment as is the case in marine I sediment, where it can be as thick as half a meter or more.

At the point, where the sulfate concentration has become as low as about I 10 ppm (redox potential Eh ^ -150 mV) methanogenesis takes a start, because

it has become thermodynamically attractive at this low redox potential. It I will be obvious, that the relative importance of methanogenesis is much " greater in fresh water sediment than in marine sediment [j3,14].

Methano-I

genie bacteria like Methanobacterium and Methanosarcina, are heterotrophic

and obligate anaerobic organisms [15,16]. Besides methanogens there are numerous fermentative micro organisms, obligate anaerobs, in the reduced I sediment, that proyide low molecular substates for sulfate reducers and

methanogens.

I There appears to be a, metabolic coupling between sulfate reducers and metha-nogens. The sulfate reducers produce low-molecular fatty acids, like acetic I acid and hydrogen, Acetic acid is a major substrate for methanogens, that

split it into methane and carbon dioxide. Methanogens also produce methane

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out of carbon dioxide and hydrogen. In fact carbon dioxide seryes an

elec-tron acceptor to these organisnjs [i3,!4,17,18,1 9,20] ,

Carbon dioxide and methane diffuse up to the overlaying water. On the way •

up a part of the methane is being oxidized to carbon dioxide by microbes

with the aid of oxygen and nitrate \jï\ . Oxidation with the aid of sulfate

does not seem to accur or proceeds very slow Q4,19] . The methane, that

diffuses into the overlaying water, is oxidized there or is released in the

air. Ebullition of methane may take place in case of very vigorous

methano-genesis [21,22]. If the lower part of the overlaying water gets anaerobic,

for instance as a result of stratification, the layered structure of the

sediment will gradually disappear. Even sulfate may become totally depleted,

in which case the sediment is completely reduced and fermentation and

methanogenesis are the remaining- decomposition processes. Methanogenesis may

even take place in the lower part of the hypolimnion [22].

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8 -AIR /•—> WATER

u

"INACT1VE' SEDIMENT ORG-C REFRACTORY

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The rate of mineralization is dependent of the concentration of particulate organic matter, its chemical composition, the temperature, the oxygen con-centration, the nutriënt concentration and the concentration of bacteria. The outmost important variable is the temperature. Temperature and composi-tion dependence of the decomposicomposi-tion rate will be dïTsciïssed seperately in section 2.2.1.3 and 2.2.].4. Models (2,2.1.1) and their parameters (2.2.1.2) will be discussed first.

2.2.1.1 Models

Several models are being used for the description of biodegradation in water systems. The most simple version is the Streeter-Phelps first order

equa-tion [23] :

~ = -k.C (2.1)

C is the concentration of BOD, COD or particulate organic matter. This model appeared to match measurements only partially. As time proceeded during decomposition the value of k turned out to decrease, The explanaticm for this seemed to be, that the substrate consists of components with different degradability. The most easy degradable components are decotnposed first. An empyrical equation, in which the rate constant is a function of time, was

used to describe this phenomenon [233 :

This is not very handy and was not satisfying either. Then it was revealed, that a part of the organic matter, from discharges as well as algae, was non-biodegradable [23,24,9]. This fraction originates during decomposition and consists of humic acids and such-like substances. Humic acids are large molecules, complicated polymers of carbohydrates, amino acids and fatty

acids. In order to make a correction for the refractive fraction (f) the first order equation was written as JJ23] :

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10

-k(C-f.C ) (2.3) o

o

Although f will not be a perfect constant in reality, this model satisfied quite well in a number of raineralization tests j_9,24J.

The model can be extended by introducing the biomass of the decomposers. If it is assumed, that the decomposer biomass is proportional to the quantity of organic matter that has been decomposed already, this can be done in the f ollowing way [$] :

~ = -k.(C-f.C ).(C -C) (2.4)

dt o o

Jewell and McCarty [jj] show, that the decomposition of an algae culture, which was grown axenically (without bacteria), proceeded according to second order kinetics after inoculation, The concentration of bacteria had to in-crease to its maximal value. However, the extension seeras to be superfluous for the description of decomposition in natural water. The generation time

yA ^ of bacteria is ten to hundred titnes longer than that of algae. This means,

^W*6* that the dynamics of bacteria are fast enougb to adjust rapidly even to

rather sudden changes in algal dynamics. In other words, the bacterial con-centration will almost always be maximal with respect to the environmental conditions.

Autolysis, a phenomenon, that seems to be important with respect to the de-composition of algal matter, is not yet included in mathematical models. lts incorporation in models tnay substantially improve the performance of these models.

2,2.1.2 Values for model parameters

Before we mention the parameter values, we must make some comments on the nature of the experiments, with which these values have been determined. The experiments with respect to decomposition of algae biomass (and nutriënt regeneration) almost always have the same set-up. An algae culture, gathered from natural water or grown in laboratory, is placed in a dark room at a constant temperature. Aeration by shaking or gas bubbling takes place in case of aerobic decomposition. At regular times samples are taken for deter-mitiation of a number of variables, like particulate organic carbon (as COD or BOD),particulate organic nitrogen and phosphorus, ammonium, nitrate and

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phosphate. The method has several disadvantages. Not only microbial decompo-sition is being measured. Autolysis of dead algal cells and respiration of still living algae are included too, Autolysis in particular will accelerate the decomposition process by the fast production of relatively small easy degradable molecules. Calculated k values will be too high. Especially in the case, that calculations are being done on the basis of particulate organ-ic carbon, so that the organorgan-ic matter, that is dissolved by autolysis, is not taken into account. Respiration of still living cells can be very sig-nificant too. Jewell and McCarty Qf] concluded fram their experiments with an axenic culture of Ch_lore_l_la pyreno'£dosa, that af ter a 100 days incubation in the dark about 30% of the algae biotnass was respired. They also found that lag-periods of maximally ten days may accur for the decomposition of cultures with very young cells, because they survive longer than cultures with older cells. Anyhow, there are many indications that algae cells can survive in the dark for a long time. This phenomenon will result in k-values, that are too low. However, the accelerating effect of autolysis seems to dominate, because it appears from analysis of literature data

[9,24], that k-values for the first 5 to 10 days of decomposition are 1.5 to 2 tiraes as high as k-values for the period after that.

Sudo a.o. [24] calculated k with equation 2.3 on. the basis of particular or-ganic matter for the first 20 days and f as the average of the remaining fractions at the thirtieth and hundredth day, The results are gathered in table 2.1. The COD of the filtrate initially increased from 7 to 10 mg/l and

1 2 3

4

5 6

7

culture species Stigeoclonium tenue Chlamydomonas sp. Chlorella sp. Selenastrum capricornutum Chlorella sp. Selenastrum capricornutum Chlorella sp. o r i g m culture effluent river river pure culture river pure culture river decomposition temperature

<°O

20 20 20 20 10 10 30

k

(.f

1

)

0,08-0,1 0,] -0,2 0,09-0,08 0,07-0,08 0,04 0,03 0,2

f

(%) 30-33 19-23 37-43 20-25 53 63 23

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12 -!

2

3

4

5 1

2

3

4

5

1

2

3

4

5

6

1

2 3

4

5

6 1 2 3 4 5 6 1 2

3

4

5

6

origin culture mixed from estuary mixed from tidal river mixed from river mixed from eutrophic lake mixed from effluent AZI mixed from river Chlorella pure culture nature of medium salt water estuary brackish water tidal river fresh water river fresh water lake effluent

AZI

synthetical river water synthetical river water duration culture growth (days)

9

16

27

54

207

14

19 22 59 211 16 18 20

24

59 211 12 16 21 24 54 211

12

16 21 24 59 21! 16 18 21

24

59 211 20 decomposition duration (days) 312 313

366

323

295

317

320 365 322 230 315 321 341 365 322

295

319

313

340

365 322

295

319 323 340 365

330

295

315

321 340

365

322

295

4-3-70

k

1

<<rt

0,100 0,034 0,019 0,010 0,011 0,150 0,034 0,010 0,010 0,025 0,140 0,052 0,040 0,067 0,014 0,021 0,080 0,034 0,054 0,032 0,014 0,021 0,028 0,041 0,026 0,021 0,030 0,015 0,055 0,016 0,042 0,057 0,049 0,011 0,015-0,070

f

(%)

70

40

34

38 56

40

48

49 34

74

60 61 26 70 34

42

86 27

50

23 34

44

37

22 19 21 40

64

33

34 35

49

45

64

aver 45

Table 2.2 Results of decomposition-experiments [9] Flagellates, diatoms and green algae were dominating in young cultures and bluegreen algae were dominating in old cultures (T 20°C)

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began to decrease af ter 100 days, The pH increa.sed from 7.6 to 9.8 in 20 days and decreased afterwards to a value between 7.1 and 7.8. Because nutriënt concentrations were high, nutriënt limitation did not occur.

Jewell and McCarty [9] did the same on the basis of total COD. Their results are presetited in table 2.2. k and f appear to be functions of culture age, k decreases as the culture gets older and reaches a more or less constant value for cultures with an age of 30 days or more. f has its highest values for very young cultures and very old cultures. This is remarkable, because one should e^pect low values for young cultures, The explanation may be the high surviving capacity of young cells. Chemical composition may play a role tpo. There seems to be no difference between the results for fresh and salt water media.

According to Jewell and McCarty [9] algae biomass contains three fractions: 1 a rather sulall easy degradable fraction of a few percents, decontposed

within a couple of hours after death of the cell. 2 a degradable fraction, 70-30%.

3 a refractory fraction, 20-60%.

It seems rea^onable to assume, that the first fraction, not only containing the low molucjular stock of the cell, but also products of autolysis, is somewhat bigger up to 10-20%. The role of autolysis becomes more clear, if we compare the results of Sudo a.o. [24] and Jewell and McCarty [9] . The former find much higher k~values due to autolysis, they calculated k for a relatively short initial period on the basis of particulate organic carbon. Fallon and B^ock [38j made decomposition experiments with uncultured samples from Lake Meridota. The algae species were all bluegreens. They calculated k-values with| equation 2.1 on the basis of total BOD for the initial decom-position perijod of 4 to 7 days. Table 2.3 represents their results.

The k-values are rather high compared to the other ones, that have been mentioned so far. This may indicate, that cultured algae are less suscepti-ble to decomposition than in situ grown algae. Application of laboratory results to na[tural water systems should be done cautiously.

Uien [_39] mad|e decomposition-experiments with algae suspensions, that were filtered from samples from Lake Norrviken and calculated k-values with equation 2.3 on the basis of particulate organic carbon (COD). The results, shown in tabql 2.4, show no significant differences between different species. The f-values look somewhat low.

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Starting Date (1976) 6 July 12 July 19 July 26 July 2 August 10 August 16 August 23 August 6 September 11 October 27 October 9 November k(d"]) 0.322 0.115 0.140 0.108 0.345 0.253 0.010 0.083 0.012 0.023 0.023 0.010 Starting Date (1977) 15 June 27 June 5 July ]J July 26 July 1 August 8 August 15 August 22 August 1 September 7 September k(d J) 0.060 0.048 0.124 0.094 0.097 0.085 0.085 0. .117 0.J38 0,060 0.154

Table 2.3. Results of the decomposition experiments of Fallon and Broek [38]. Bluegreen algae were dominating. The teroperature was 20°C.

dominating species Green algae Oscillatoria Oscillatoria Green algae Diatoms Oscillatoria sampling date 23 June J97O 2 October 1973 2 October 1973 30 May 1974 12 May 1975 28 October 1975 k(d"]) 0.028 0.049 0.049 0.071 0.056 0.020

f

0.J9 0.17 0.J3 -0.17 0.29

Table 2.4. Results of the decomposition experiments of Ulën [39] The temperature was 10 C,

2.2.1.3 Decomposition rate and temperature

Streeter and Phelps found out, that the temperature dependence of the rate constant k could be represented by the Van ft Hoff-Arrhenius equation [25].

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E is the activation energy and R, the gas constant (8.36 J/mole. C ) . Within the temperature range, occurring in natural water, this equation can be sim-plified to

(2.6)

This is the usual formula for representation of temperature dependence of biological processes.

Fair a.o. [25] give a number of values for C, . The mean value of C, for ae-robic degradation is 0.07°C~1 ( eC k = 1 . 0 7 ) . <\\ --~±

Sudo a.o. [24] used equation 2.5 for the determination of E from the data in table 2.1. They found that E = 58.8 kJ/mole C for Chlorella s£. and that E = 42.0 kJ/mole C for Selenastrum capricornutrum in the temperature range of JO-30 C, They also concluded, that f may be temperature dependent, f ap-pears to increase with decreasing temperature in case of Chlorella decompo-sition, but stays more or less constant in case of Selenastrum decomposition. Jewell and McCarty \sj also find an increasing f for decreasing temperature. They find for Chlorella that f is U - 1 6 % , 32-39% and 46% at respectively 35, 25 and 20 C, and that k increases a factor 1.5-2.5 per 10 C temperature in-crease.

2.2.1.4 Decomposition rate and chemical coroposition

It can be seen in tables 2.1 to 2.4, that substantial differences may occur in k and f between different algae species and even between different cultu-res of the same species. These differences are probably strongly related to the varying chemical composition, which in its turn is dependent of the species and the growth conditions (growth rate).

Roughly the algae biomass consists of three components; proteins, lipids and carbohydrates. Very little research for a relatioh between decomposition rate and the proportions of these three components in algae biomass has been carried out.

Cranwell [_26J finds the following in the decomposition of two bluegreen algae Gloetrichia echinulata and Oscillatoria agardhii. Oscillatoria with

10-11% lipids is decomposed substantially faster than Gloetrichia with 4-5% lipids. The percentage of lipids does not change too much during

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decomposi 16 decomposi

-tion.

Koyama a.o. [lQ deduce from the ratio's of the fractions of pigraents, fatty acids, amino acids, carbohydrates and humic acids at several depths in the, sediment of two lakes, that the decomposition rates of these fractions are arranged in the following order of magnitude:

pigment > fatty acid > amino acid > carbohydrate > humic acid. However, the anaerobic decomposition is playing a role here too.

From these scarce data only very preÜminary conclusions may be drawn. The aerobic decomposition of organic matter seems to be retarded in case of an increasing content of carbohydrates and to be accelerated in case of an in-creasing lipid and/or protein content, This conclusion finds support in the data of table 2.2. The young cultures, containing more lipids and proteins and less carbohydrates than older cultures, have much higher k-values than these older cultures.

2.2.2 Aerobic mineralization of dissolved organic carbon (DOC)

The definition of DOC is not unambigious. However mostly it is defined as the organic matter, that passes a 0.5-0.1 y filter. This means, that DOC consists of colloidal matter too [jöl. DOC can enter a natural water system in different ways. It may be excreted by algae. It may originate from auto-lysis and microbial decomposition or it raay arise from organic pollutant containing discharges.

The concentrations of DOC, that have been measured, vary a lot and show a seasonal fluctuation. Mostly they are between 0,5 and 7 mgC/1 in fresh and salt water. The concentrations in fresh water are usually higher than in

salt water [6,7,10].

DOC is generally divided in two fractions; one that is readily degradable and one, that is very resistant to biodegradation. The latter is usually about 50% of the total dissolved organic carbon concentration and consists largely of humic-like substances, that have been mentioned before.

Ogura [6,7] has carried out a large number of decomposition experiments with DOC. Decomposition took place in the dark at a temperature of 20 C. DOC was measured as COD. The decomposition rate constants were determined for each fraction with a simple first order model. The results are gathered in table 2.5.

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origin of sample

Tokio Bay (salt) Sagarai Bay (salt) Mean value

River water (fresh)

k/d"1) 0.11-0.033 0.095 0.082 0.087 k2(d"') 0.0085-0.0027 0.0047 0.0047 0.0064

Table 2,5. Decomposition rate constants for the two fractions of DOC [6,7] .

2.2.3 Methane oxidation

Metbane is alraost always only produced ïn the sediment. If the overlaying water is mixed more or less homogeneously, oxidation of methane- takes place in upper layer of the sediment in the presence of oxygen and/or nitrate and in the overlaying water. In case of stratification oxidation of methane takes mainly place in a relatively thin water layer (<, 1 m) at the depth where the oxygen concentration goes to zero. This layer is not necessarily

situated in the metalimmion [22,27,28].

Methane oxidation can put a very substantial claim at the oxygen budget. Rudd and Hamiiton [22] calculated for Lake 227 (Canada), that about 60% of all produced tnethane was oxidized and 40% was released into the air. Because

13% of the primary production was transformed into methane, about 8% of primary production was reoxidized to carbon dioxide with methane as inter-mediate: Higher percentages have been found for other lakes f29].

2.2.3.1 Methane oxidation rates

Rudd and Hamiiton [28] studied the kinetics of methane oxidation in Lake 227. They applied the Michaelis-Menten model (see 2,3.1) on the results of in vitro-experiments with methane concentrations from 0.37 to 38.3 yiM, that are representative for natural waters. The model matched the results well, and yielded the following results (at T = 6°C and [oo"l = 0 . 7 ppm): V = 0 , 3 2 uM/h

and K =* 4.7 yM.

Sansone and Martens [2f] calculated for in vitro-experiments with methane concentrations of 0.1 - I . 3 pM in salt water, that k = 0.017 - 0.004 yM/d

(dissolved inorganic nitrogen (DIN) < 14 yM, 0„ = 0.] - 0.2 ppm) and k = 0.24 - 0.176 yM/d(DIN > 14 ]M, 02 » 0.ï ppm) at 25°C. They found in situ

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18

-k-values of 0.053 jjM/d (DIN is very low, 02 = 0.1 - 0.01 ppra, CH4 = 0.7

-0.] ]jM). They found clear correlations between k and DIN and between k and the initial concentration of methane.

A first order k can be estimated from concentration and oxidation rate pro-files given by Rudd a.o. [27] for Lake 120, The highest oxidation rate can be found in the micro-aerobic layer; k = 1.27 d (T = 4 C, 0„ = 0.3 ppm» CH4 = 3 liM).

The same can be done for in situ data, measured by Howard a.o. j_30] in Lake Erie; k - 0.08 d~1 (T = 25°C, O2 is unknown but probably high CH^ = 440

y.M)-This k-value is about one tenth of the value, that was deduced from the data of Rudd a.o. [27] in spite of the much higher temperature. The explanation may be found in substrate inhibition and/or inhibition by oxygen (see next

section).

2.2.3.2 Oxygen inhibition of methane oxidation

Methane oxidizers seem to be micro-aerophile [21,27j. Indications for inhi-bition by oxygen may be found in the following. Methane concentrations in the epilimnion mostly were low but clearly detectable (0.5-3 y M ) , while no or very slow oxidation took place [22,28,30]. IÊ the oxygen concentration was increased above 1 ppm, the methane oxidation rate decreased to about one

tenth of the original rate [28]. From data of Rudd and Harailton [22] can be calculated with a first order model, that relatively low k-values (0.3-0.033 d ) occur in Lake 227 after turn-over. These arguments strongly indi-cate inhibition by oxygen above the concentration of 1 ppm. Methane oxi-dizers have been proved to concentrate in the micro-aerobic layer o£ the hypolimnion.

2.2.3.3 Methane oxidation rate and temperature

The optimum of methane oxidation appears to be at 25 C, Methane oxidation is not a particular steep function of temperature, Methane oxidizers are psy-chrophile (more or less equally active in a rather broad temperature range). Rudd a.o. [28] found an activation energy of 38 kJ/mole in a temperature range of 2-22°C, which is equal to a C value of 0.058 °C .

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2.3 Anaerobic' mineraliaation

Anaerobic mineralization occurs in the absence of oxygen. The role of the electron acceptor is subsequently taken over by nitrate, sulfate and carbon dioxide. Mineralization by denitrification and sulfate reduction will not be discussed here. They proceed analogous to aerobic decomposition but with a reduced rate. When nitrate has been depleted and the sulfate concentration has become low (< 10 ppm), methanogenesis, which will be the issue of this section, will take over. Generally methanogenesis takes place in the sedi-ment .

The contribution of methanogenesis to mineralization is very substantial in fresh water lakes, Strayer and Tiedje [29] calculated, that 33-39% of the primary production is transformed into tnethane in Wintergreen Lake. Rudd and Hamilton [22]] calculated this part to be 13% in Lake 227. In case of strati-fication hypolimnetic methane concentrations can increase to 2500 yM [}\] . Even saturation with methane may occur, after which ebullition of methane takes place.

2.3.1 Models

To methanogenic decomposition in water the same first order model, represen-ted by equation 2.3, has been applied as to aerobic decomposition.

Kinetic studies of methanogenic decomposition in sediment have been done with zero order, first order and Michaelis-Menten models. The MM-model looks as follows;

42 * -v J L

dt m " K+C

(2.7)

V is the maximal conversion rate for substate C. K. is the half saturation concentration, a concentration for which the conversion rate has half its maximal value.

2.3.2 Metabolism of methanogenesis

The methanogenic anaerobic decomposition proceeds in three steps Q s l - The first step is formed by hydrolysis of the large organic molecules like carbo-hydrates (cellulose), proteins and lipids by extra-cellular enzytnes like cel-lulases, proteases and lipases. The second step is the degradation of the

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20

-hydrolysis products', glucose, amino acids and fatty acids to low molecular

carbolic acids (mainly acetic acid), carbon dioxide and hydrogen, The

con-version of sugars and amino acids occurs in the glycolysis. Besides fhis,

amino acids are directly transformed into carbolic acids by way of the

strickland-reaction [ló]:

R - C - GOOR + 2H2O •+ R - COOH + OT3 +

H

The carbolic (or fatty) acids are degraded by way of 3~oxidation to mainly

acetic acid [j6]. These first two steps are carried out by sulfate reducers

and fermentative obligate anaerobic bacteria, like Clostridia. The third

step is production of methane by methanogens mainly from acetic acid and

from carbon dioxide and hydrogen. The reaction equations are the following

[11,13,14,16,17,19] :

, - COOH

C 0

2 + 4 H

2 "* G H

4 + H

2 °

It has been shown in numerous experiments, that acetic acid and carbon

dioxide/bydrogen are by far the main substrates for methanogenesis.

Cappenberg [2(f[ calculated, that 70% of the methane, produced in the

sedi-ment of Lake Vechten, was coming from acetic acid. The remaining 30% came

from carbon dioxide. Winfrey and Zeikus [32] found 41% for the latter

frac-tion in Lake Mendota sediment. Koyama [l Q calculated for mud of

paddy-fields respectively 60 and 40% for the two fractions. Smith and Mah [33]

find 70 and 30% for anaerobic fermentation of carbage with active sludge.

These data strongly suggest, that the percentages of 70 and 30% are quite

universal.

A number of alternative substrates for methanogenesis have been suggested in

literature. It has been shown, that methane can be produced from methanol

and formic acid. There is no certainty about ethanol, butylic acid, propionic

acid and a number of less important substances Q>,11,J3,29J. However, these

substrates play a very minor role in the sediments of natural waters.

Methanogenesis has its optimal pH between 6.7 and 7.4 and is seriously

lim-ited, when the pH drops below 5.5. Q>»'-Öf Koyama [l f] reports inhibition of

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methanogenesis,by salt (NaCl) concentrations of the magnitude occurring in

seawater (1000-1200 meq/1). It concerned a fresh water population, so that

it cannot be said, that salt water populations too are inhibited by high

salt concentrations.

2.3,3 Inhibition by sulfate

Methanogenesis is inhibited in the presence of sulfate. In the upper part

(40 cm to more than a meter) of -marine sediment with very high

concentra-tions of sulfate (z 2000 ppm) methanogenesis is severely suppressed. The

sulfate concentration in fresh water mostly is not much higher than 20 ppm.

Methanogenesis will be only slightly inhibited in fresh water sediment.

However, sulfate reduction and methanogenesis do not exclude each other, as

previously has been thought. Low methane production rates have been measured

in a salt tidal tnarsh with rather high sulfate concentrations [35].

Inhibi-tion experiments showed only little inhibiInhibi-tion, when a sulfate concentraInhibi-tion

of 20 ppm was present [)T\ •

Cappenberg Tl 3] found, that sulfate reducers were concentrated raainly near

the surface of the sediment in Lake Vechten and, that methanogens were

con-centrated mainly in the sediment layer between 3 and 5 cm below the surface.

However, both were present in the whole active layer, although it must be

remarked, that the sulfate concentration was undetectably low.

In literature three possible mechanisms of inhibition are distinguished;

In-hibition by sulfide, that is produced in the reduction of sulfate,

inhibi-tion caused by a high redox potential (Eh), and inhibiinhibi-tion caused by

sub-strate competition. Sulfide is recognized as potential inhibitor by

Cappenberg []3,1<T|. Several experiments, however, have shown, that a sulfide

concentration below 0.3 mM even stimulates methanogenesis [17,36]. This can

be explained by decrease of the redox potential. The redox potential

de-creases if the ratio S/SO, inde-creases, which makes methanogenesis more

attractive from the termodynamical point of view. Inhibition only occurs

above a sulfide concentration of 3.2 mM, rarely realized in natural sediment

[14,36],

Substrate competition in combination with a relatively high redox potential

seems to be the most probable ntechanism of inhibition, which may be

support-ed by the following. In spite of inhibition of methanogenesis in the

pres-ence of sulfate there exists a clear metabolic coupling between sulfate

re-ducers and methanogens. Sulfate rere-ducers do not only produce acetic acid,

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to rt

g-

O O O 3 cm o Hl O) c H l I-j n> & o rt O

g

SULFATE ftEDUCERS OBLIGATE ANAEROBIC BACT HYDROGEN 1 S

g

(0 to H- CO

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tbat sulfate reducers have a net production of hydrogen in the absence of sulfate and a net consumption of hydrogen in the presence of a sufficiently high gulfate concentration £l3,14,17[[. Strayer and Tiedje J_29j concluded, that more hydrogen was consuraed than was necessary for methanogenesis. This indicates an additional destination for hydrogen. It is also suggested,

that there exists a third group of bacteria, which produces hydrogen ^14,15j. With the addition of tetra, fluor acetic acid \j9J, hydrogen or acetic acid, substrate competition has been shown to occur indeed. Especially the experi-ment, in which the inhibiting effect of sulfate was overridden after

addi-tion of acetic acid or hydrogen,, made this clear.

The relation between substrate competition and redox potential becomes clear as follows. Principally the oxidation of acetic acid and hydrogen by sulfate is thermodynamically more attractive than methanogenesis from acetic acid or carbon dioxide and hydrogen. The latter process will only get the upper hand, when the redox potential has become sufficiently low. Methanogenesis already occurs at a redox potential of -lOOmV, but will only be optimal at -300mV £l3j. The metabolic coupling between sulfate reducers and methanogens is presented in figure 2,2. The scheme also indicates, that sulfate reducers are able to oxidize methane. In a number of experlments this could not be proved j]i4,17]. However, Desulfovibrio desulfuricans has been shown to be capable of the oxidation of methane at a very low rate ]j9,34].

Bernard [4oJ succesfully incorporated first order methane oxidation in a

methane diagenesis model for marine sediment. He used a first order rate constant i 1 *1 1

of 0.156 x 10 d (I » 4.5 °C) mentioned another value of 0.69 x 10~ d (T = 15 C ) , given in literature, and owed the difference to the lower sediment temperature in his study.

2.3.4 Values for model parameters from decomposition experiments

Foree and McCarty [5] made anaerobic decomposition experiments with cultured algae, that were very similar to the decomposition experiments described in section 2.2.1.2. In this case the medium was blown through with nitrogen.

Respiration is not taking place anymore, but autolysis is still very important. The algae (mixed or pure culture) were cultured during 30 to 50 days and were decomposed at 20 C. k-values, calculated with a first order model with cor-rection for a refractory fraction on the basis of particulate COD and for a period of 160 days, ranged from 0.011 to 032 d~ . The average value of f was 0.4 with a Standard deviation of 0.15. These k-values show little difference with those for aerobic decomposition. The fact,that autolysis is

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24

-very important,appears from deconjposition experimenta with an axenic culture of Chlorella. pyrenoïdosa. Particul,ateorganic ca.rbon decrea,sed with only half the rate compared to inoculated cultures. Decomposition experiments with inocula of different strength showed no significant difference with respect to the rate of particulate COD decrease. But differences in rates of total COD decrease were substantial.

Otsuki and Hanya [4 ij found the following parameter values for the decompo-sition of a Scenedesmus species culture (age 6 days) at 20 C; k = 0.02 d (simple first order model), f = 0,4.

2,3.5 Values for model parameters from sediment experiments

A number of experiments have been performed, in which turnover rate con-stants were determined for acetic acid, hydrogen and carbon dioxide, and that cast a light on the rate of and rate limiting step in methanogenesis in sediraents.

Winfrey and Zeikus [3Q determined the following first order turnover rate constants in Lake Mendota sediment at 10°C: k = 4.5 hr (AC = 2.7

-, AL

4.5 JJM/1 wet sediment), kn_ » 0.009 hr (C0„ = 6.4 - 8.3 mM/1, H„ < 3 u M ) .

It must be remarked, that compaction of about 30% occurred during sediment sampling, wbich influenced the results. The authors suggest, that the rate limiting step in methanogenesis is the production of acetic acid and hydro-gen, because both have very low concentrations. This tneans in fact, that the degradation of the large organic molecules is rate limiting.

Cappenbérg [jïol found the following turnover rate constants in the sediment of Lake Vechten at 10°C: kA C = 0,35 hr~ (AC = 40 yM/1 wet sediment). Acetic

acid should be converted in methane for 86%. Oxidation of acetic acid will have been very slow indeed, because the sulfate concentration was undetect-ably low. The difference existing between the k -values found in Lake

Men-AO

dota and Lake Vechten sediments may be explained by a much faster sulfate reduction in the former.

Cappenbérg mentiones two other k -values found in different anaerobic -1 l systerns: kA C = 0.45 and 0.3 hr . Sraith and Mah [33] found kA C = 0.312 hr

in anaerobic active sludge, It appears from the data, that k is rather AO

constant in a purely anaerobic system,

Strayer and Tiedje [29] applied MM-kinetics to the sediment of Wintergreen Lake and found the following parameter values at 10-14 C: Conversion of hydrogen; K = .1.2 - 4,0 }jM/l wet sediment and V = 300 - 750 yM/l.hr (H„ =

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0.2 - 0.7 )JM/1). Methane production; K * 3.5 ]JM a.nd " 135.6

CH,/l.hr, Conversion of acetic acid; K =• 20 - 34 jjM/1 and Vffl B 20 ]4M/l.hr

(AC = 1 8 - 5 5 yM/1)„ If we transform MM-kinetics into first.order kinetics, we find k = 0.6 hr . These figures may be somewhat too high, because

quite substantial quantities of acetic acid and hydrogen were added to the sediment samples.

Some overall metbane production rates have been given in literature for the sediment of a number of lakes, They are brought togetber in table 2.6.

lake Lake Vechten Lake 227 Lake 227 Wintergreen Lake reference 20 22 22 31 methane production (mM/m .d) 33.6 (yearly average) 10.5 (stratified) 4.8 (unstratified) 10-46 (eutropbic)

Table 2.6. Overall metbane production rates

2.3.6 Decomposition rate and temperature

The optimal temperature of methanogenesis lies between 35 and 42 C

Foree and McCarty VS] have calculated, that C, *= 0.055 C for the decompo-sition of algae in the temperature range of 15-25 C. Fair a.o. [[25] found a substantial higher figure for the anaerobic gas production in sediment of a lake in the temperature range of 4-21°C : C = 0.095°C~ . The in situ

methane production may, however, increase more froxn winter to summer, than would be calculated with these C -values. Zeikus and Winfrey [37J found a hundred fold increase of methanogens from winter (4 C) to summer (J6 C) in Lake Mendota sediments. They tneasured production as a function of

tempera-ture. It can be calculated from their production curves, that C, =

0.18 - 0.24°C~ for sediment sampled in winter and Ck = 0.059 - 0.1°c" for

sediment sampled in summer in the temperature range of 4-35 C. The former C, -values, that are extremely high, indicate an increase in methanogens.

2.3.7 Decomposition rate and chejnical cotnposition

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decompo-I

26

-sition rate constants of cultures of different algae species (greetts and

bluegreens). What they did find, was a correlation between decomposition

rate and lipid content of the culture, The lipid content may vary wlthin

the same species from 10 to 60%. It appeared»that the decomposition rate

decreased, when the lipid content increased. This hypothesis is supported

by the change in chemical composition. of the particulate matter during

decomposition, Before decomposition the composition was 26,6% proteins, 3J%

lipids and 62.4% carbohydrates. The cotnposition of the remaining refractory

matter was 37.3% proteins, 14% lipids and 48.7% carbohydrates. This points

in the direction of the conclusion, that proteins and lipids are degraded

less fast than carbohydrates under anaerobic conditions.

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Literature

1 SEKI, H., Relationship between production and mineraHEation of organic matter in Ahuratsubo Inlet, Japan.

Journal of the Fisheries Research Board of Canada, 25 (1968), 625-637.

2 PAERL, H.W., Bacterial sediment formation in lakes; trophic implications. In: Interaction between sediments and fresh water, Proc. Int. Symp., Golterman, H.L., (ED), Amsterdam, September 1976, pp. 40-47.

3 WRIGHT, R.T., HOBBIE, J.E., The uptake of organic solutes in lake water Limnol. Oceanogr. J_0 ('965), 22-28.

4 VACCARO, R.F., JANNASCH, H.W., Variations in uptake kinetics for glucose by natural populations in seawater.

Limnol. Oceanogr. J_2 (1967), 540-542.

5 FOREE, E.G., McCARTY, P.L., Anaerobic decomposition of algae. Environ. Sc. Technol, 4_no. 10 (1970), 842-849.

6 OGURA, N., Rate and extent of decomposition of dissolved organic matter in surface seawater.

Marine Biology j_3 (1972), 89-93.

7 OGURA, N., Further studies on decomposition of dissolved organic matter in coastal seawater.

Marine Biology JÏJ_ (1975), 101-111.

8 PAERL, H.W., Microbial organic carbon recovery in aquatic eco-systems, Litnnol. Oceanogr. 2^3 no. 5 (1978), 927-925.

9 JEWELL, W.J., McCarty, P.L., Aerobic decotnpbsition of algae. Environ. Sc. Technol. 5_ no. 10 (1971), 1023-1031.

10 BADA, J.L., LEE, C , Decomposition and alteration of organic components, dissolved in seawater.

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28

-Literature (continued)

1! KOYAMA, T., a.o., Decomposition of organic matter in lake sediments.

In: Proc. of the symposium on Hydrochemistry and Biochemistry, Ingerson,

E. (ED), _2 (1973), 512-535.

12 KESSEL, J.F. van, Influence of denitrification in aquatic sediments on

the nitrogen content of natural waters.

Thesis, Landbouwhogeschool, Wageningen, The Netherlands, 1976.

13 CAPPENBERG, Th.E., Interrelation between sulfate-reducing and

methane-producing bacteria in bottom deposits of a freah water lake I: Field

observations.

Journal of Microbiology and Serology 4£ (1974), 285-295.

14 OREMLA1SD, R.S., TAYLOR, B.F., Sulfate reduction and methanogenesis in

mar ine s edimen t s.

Geochimica et Cosmochimica Acta hl^ (1978), 209-214.

15 TOERIEN, D.F., HATTINGH, W.H.J., Anaerobic digestion I; The

microbio-logy of anaerobic digestion.

Water Research 3_ (1969), 385-416.

16 ANDREWS, J.F., PEARSON, E.A., Kinetics and characteristics of volatile

acid production in anaerobic fermentation processes.

Int. J, Air Water Poll. 9^ (1965), 439-461.

17 WINFREY, M.R., ZEIKUS, J.G., Effect of Sulfate on Carbon and Electron

flow during microbiol Methanogenesis in Fresh water Sediments.

Applied and Environmental Microbiology _33 no. 2 (1977), 275-281.

18 SOROKIN, Yu.I., Experimental investigation óf bacterial sulfate-reduction

35

in the Black Sea u s m g S

Mikrobiologiya 3± (1962), 402-410.

19 CAPPENBERG, Th.E., Interrelations between sulfate-reducing and

methane-producing bacteria in bottom deposits of a fresh water lake II: Inhibition

experiments.

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Literature (continued)

20 CAPPENBERG, Th.E., Interrelations between sulfate-reducing and

methane-producing bacteria in bottom deposits of a fresh water lake III:

Experi-14

• ments with C-labelled substrates.

J. Microbiol. Serol. 40 no. 3 (1974), 457-469.

21 SANSONE, F.J., MARTENS, C.S., Methane oxidation in Cape Look-out Bight,

Nortb Carolina.

Limnol. Oceanogr. 2 3 n o , 2 (1978), 349-355.

22 RUDD, J.W.M., HAMILTON, K.D., Methane cycling in an eutrophic shield lake

and its effects on whole lake tnetabolism.

Limnol. Oceanogr. £3 no, 2 (1978), 337-348,

23 STONES, T-, Kinetics of biocheiaical oxidation of sewage.

Effluent and Water Treattnent Journal JJ9 (1979), 28-30.

24 SUDO, R., AIBA, H.O.S., MORI, T., Some ecological observations on the

decomposition of periphytic algae and aquatic plants.

Water Research J_2 (1978), 177-184.

25 FAIR, G.M., M00RE E.W., THOMAS, H.A., The natural purification of river

muds and pollutional sediments IV and V.

Sewage Works Journal J_3 no. 4 (1941), 756-779.

26 CRANWELL, P.A., Decomposition of aquatic biota and sediment formation;

Liquid components of two blue-green algae species and of detritus

resul-ting from micriobiol attack.

Fresh water Biology 6_ (1976), 481-418.

27 RUDD, J.W.M., HAMILTON, R.D., CAMPBELL, N.E.R., Measurement of microbiol

oxidation of methane in lake water.

Limnol. Oceanogr. }9_ no. 3 (1974), 519-524.

28 RUDD, J.W.M., HAMILTON, R.D., Factors controlling rates of methane

oxida-tion and the distribuoxida-tion of methane oxidiaers in a small stratified lake,

Arch. Hydrobiol. 25.no. 4 (1975), 522-538.

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30

-Literature (continued)

29 STRAYER, R.F., TIEDJE, J.M., Kinetic parameters of the conversion of

methane precusors to methane in a hypereutrophic lake sediment.

Appl. Environ. Microbiol. 36_ no. 2 (1978, 330-340.

30 HOWARD, D.L., FREA, J.I., PFISTER, R.M., The potential for methane-carbon

cycling in Lake Erie.

Proc. 14th Conf. Great Lakes Research (1971), 236-240.

31 STRAYER, R.F., TIEDJE, J.M., In-situ methane production in a small

hyper-eutrophic hard-water lake: Loss of methane form sediraents by vertical

diffusion and ebullition.

Limnol. Oceanogr. £3 no. 6 (1978), 1201-1206.

32 WINFREY, M.R., ZEIKUS, J.G., Anaerobic metabolism of Immediate Methane

Precursors in Lake Mendota.

Appl. Environ. Microbiol. _37 no. 2 (1979), 244-253.

33 SMITH, P.H., MAH, R.A., Kinetics of acetate metabolism during sludge

digestion.

Appl. Microbiol. J_A (1966) 368-371.

34 MARTENS, C.S., BERNER, R.A., Methane production in the interstitial watera

of sulfate-depleted marine sediments.

Science J_8_5 (1974), 1167-1169.

35 WHELAN, T., Methane, carbon dioxide and dissolved sulfate from

intersti-tial water of coastal marsh sediments.

Estuarine and Coastal Marine Science _2 (1974), 407-415.

36 WINFREY, M.R., ZEIKUS, j.G., Microbial methanogenesis and acetate

meta-bolism in a meromictic lake.

Appl. Environ. Microbiol. 2 1 no. 2 (1979), 213-221.

37 ZEIKUS, J.G., WINFREY, M.R., Teraperature limitation of methanogenesis

in aquatic sediments.

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Literature (continued)

38 FALLON, R.D., BROCK, T.D., Decompogition of Bleu-green algae (cyano-bacterial) blooma in Lake Mendota, Wisconsin.

Appl. Environ. Microbiol. .37.no. 5 (1979) 820-830.

39 UlÜN, B., Seston and sediment in Lake Norrviken, II Aerobic decomposi-tion of algae.

Schweiz. Z. Hydrol. 40 no. 1. (1978) 104-118.

40 BERNARD, B.B., Methane in marine sediments. Deep-sea Research, 26A (1979) 429-443.

41 OTSUKI, A.j HANYA, T., Production of dissolved organic matter from dead green algal cells, II Anaerobic microbial decomposition.

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I

32

-I

I 3 Mineralization of organic nitrogen, nitrification and denitrification

• This chapters deals with the nitrogen cycle and most of the important processes in that cycle, Section 3,1 gives a short discuasion of the nitrogen cycle to

cast alight on the coherence of the different processes. Ammonifications

nitrification and denitrification are respectively discussed in sections 3.2, 3.3 and 3.4. Nitrogen fixation will not be discussed separately, because it is thought, that it is quantitatively less important and a part of the pri-mary production, which is not the subject of this research.

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3.1 The nitrogen eycle

The nitrogen cycle is partially analogous to the carbon cycle, which has been described in detail in chapter 2 section 1. For this reason a rather

short description will be given here. Accent will be put upon those pro-cesses, that characterize the nitrogen cycle. Figure 3.1 gives a picture of the nitrogen cycle in an aerobic water-sediment systera.

3.1.1 Primary production of organic-N

Nitrogen is present in natural water, primarily in the shape of ammonium, nitrate and elemental nitrogen. Algae produce organic-N, mainly protein and nucleic acids, from ammonium and/or nitrate. When concentrations of nitrate and ammonium are very low, some bluegreen algae are able to produce organic-N from elemental nitrogen. This process is called nitrogen fixation.

Some bacteria are able to fix nitrogen; phototrophic and some heterotrophic bacteria, like azotobacter in the water and some anaerobic bacteria, like

clostridium in the sediment. However, their contribution to total fixation seems to be small [l,2,3,4].

3.1.2 Mineralization in water

Organic-N, produced by algae, is, after the death of the algae, decomposed to ammonium. The decomposition process is mediated by autolysis and micro-bial metabolism and is called ammonification. Ammonium is partially oxidized to respectively nitrite and nitrate, a process called nitrification.

Nitri-fication is carried out mainly by autotrophic bacteria £l,3,5].

The dead particulate organic matter settles. During settling the particulate matter becomes less rich of organic-N, which proves that nitrogen is being regenerated at a higher rate 'than carbon.

3.1.3 Mineralization in sediment

The particulate organic-N, that has settled in the sediment, is being trans-ported downwards and ammonificated until a refractory part remains.

Ammonification in the upper layer takes place aerobically, in the lower layer anaerobically, The ammonium, that is produced in ammonification, is

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partially nitrificated in the aerobic surface layer. The other part is trans-ported to the overlying water. Partially decomposed small molecular organic-N leaves the sediment too. Litte information is available about the size of the flux. There are indications, that it is not insignificant [ö] ,

At the depth, where oxygen gets depleted, nitrate replaces oxygen as electron-acceptor in the mineralization process. Nitrate is reduced to elemental ni-trogen via nitrite and a nuitiber of less important intermediates. This pro-cess is called denitrification, which takes place only, when the redoxpoten-tial has fallen below 300 mV. In denitrification a small part of nitrate is immobilized in bacterial organic-N and released as aramonium. Denitrifi-cation is carried out by heterotrophic bacteria, with exception of Thio-bacillus denitrificans, which is autotrophic [l,5,7,8]. In case of anaerobic conditions in the overlying water, denitrification takes place in the anae-robic water too [9] .

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AIR WATER

\

"INACTIVE" SEDIMENT

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36

-3.2 Ammonification

Ammonification is the process, by which heterotrophic bacteria transform

organic-N into ammonium. Bacteria are able to do this with the aid of

exoenzymes, that raay be attached to the cell wall or not, in the case of

complex organic molecules and by uptake of small organic molecules, that

contain nitrogen. A part of substrate is used for the construction of cell

material. The other part is dissimilated and released as carbon dioxide and

airanonium. The extent of atnmonium release is determined by the C/N ratio of

the substrate p ] . In anaerobic ammonification a bigger part of the ingested

substrate is dissimilated compared to aerobic ammonification. The reason

for this is, that anaerobic dissimilation has a smaller gain of energy.

Despite of slower growth, anaerobic bacteria can produce airanonium at the

same rate (or even higher) as aerobic ones [lO].

Organic-N in natural water comes from excretion of living algae, from

auto-lysis and microbial decomposition of dead algae (and other organics, that

are not considered here) and from external loads. Organic-N is particulate

and dissolved. Remineralization rates of organic-N are dependent of the

season, that means temperature dependent. Temperature dependeticeof

ammonifi-cation is about the same as that of carbon remineralization (chapter 2 section 2.3),

It is not discussed here separately because of the lack of data. Primary

production and remineralization are coupled strongly, which means that the

rates have the same order of magnitude [il, 12].

Ammonification, started in the water, proceeds in the sediment after settling,

mainly in the upper 10-20 cm [12, 13]. Because of accumulation of organic-N

and low transport rate in the sediment, concentrations of ammonium and nitrate

may be many times (10-100) higher than in the overlying water |J2, 14].

3.2.1 Ammonification in natural water

The experiments, carried out for the observation of nutriënt regeneration, have

the same set up and because of that the same disadvantages as described in

2.2.1.2. Several models are used to calculate rate constants from the results.

These models and the calculated rate constants are given below. Aerobic and

anaerobic decomposition in fresh or salt water are discussed together, because

there seem to be no significant differences from the kinetic point of view.

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3.2.1.1 Simple first-order model

The simple first-order model looks like equation 2.1, given in chapter 1 section 2.1. It is used to calculate k-values from the results of the ex-periments mentioned in this section.

Fallon and Broek [15] calculated k-values for aerobic decomposition of par-ticulate proteïn of blue-green algae» gathered from Lake Mendota, in the laboratory at 20 C. Samples are from different dates, spread over the summers of two years. They find values ranging from 0.021 - 0,12 d with a mean of 0,054 d . The rather wide range of the values points at a large variation of growth conditions, resulting in the same large variation in chemical com-position of the cell.

Otsuki and Hanya jjö] made decomposition experiments with freeze dried green algae, Scenedesmus species cultured during six days at 28 C in fresh water medium. Decomposition took place at 20°C after inoculation with bacteria

from mud. The k-value for decoraposition of particulate organic-N was

k = 0.068 d , a value almost two times as high as the value for decomposition of particulate organic-C. The simple-first order model was not satisfying. So the authors developed another model, whïch will be discussed further on. Mills and Alexander [l7] made fresh water decomposition experiments with 4 different green algae, cultured during 14 or 28 days under bacteria-free

(axenic) conditions at 20 C, Decomposition took place under aerobic condi-tions at a shaking machine at a temperature of 30 C, after inoculation with bacteria from sewage. Ko model calculations have been done, but some more general information is given. The authors find, that nutriënt regeneration of the !4-days-cultures is consequently about two times faster than that of the 28-days-cultures. This again points at the importance of growth con-ditions and the C/N ratio of the cells which must have been higher for the

23-days-cultures. The C/N ratio of the particulate matter increases from 5.4 - 5,7 to 7.3 - 7,9 during decomposition, which proves that organic-N decomposes at a higher rate than organic~C does. At the time, that decompo-sition stops, there still is particulate organic-N. The refractory part of the original particulate organic-N is about 30%. The authors have made some decomposition experiments without the presence of bacteria. The results of those experiments indicate, that autolysis may play a very important role with respect to nutriënt regeneration. Nutriënt regeneration in axenic con-ditions sometimes took place almoBt as fast and almost to the satne extent as in the presence of bacteria.

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